I do plan on going full frame in the future (within two years), so that makes the choice of lenses more complicated.

I do like how Ew mentioned that the environment I shoot in should be considered. That never crossed my mind. I live near Atlanta, so that's where I do most of my street. I think I'll rent both and see which one I like most.

Another thing to note with these lenses; due to their small size, both will make lighter aps-c cameras tip and sit unnaturally on the edge of the lens. But, with the hood attached this problem goes away entirely; so it is definitely worth buying the hood for these.

If you are renting primarily to test focal length, you might want to save yourself a few bucks and force yourself to shoot at 24mm and 28mm for a day each on your 17-40 to get an idea what focal length works best for you.

Also, FYI, in the focal length you are looking at (24-28mm) I have owned the following, the ones in bold/italics are the ones I still own currently:EF 24mm f/1.4L II USMEF 24mm f/2.8 IS USMEF 28mm f/2.8 IS USMEF-S 17-55mm f/2.8 IS USMEF-S 18-135mm f/3.5-5.6 IS STMEF 16-35mm f/2.8L II USMEF 24-70mm f/2.8L II USM

Ironically, I no longer own either of the ones you are looking at, but that is mainly due to selling my crop camera and acquiring the 24L, but I did put in time with each of them in FF & APS-C. Of all of these I've owned, the most underwhelming were the two EF-S lenses. The 17-55 was fast but the color/contrast seemed poor to me; the 18-135 was a nice focal range but just too slow.

I actually miss the feel and focal length of the 28mm f/2.8 IS on APS-C; it is such a nice fit for a small camera for street photography; some of my best shots on APS-C were with the 28 f/2.8 IS. But, it is hard to justify owning on FF when a 24+35 offers much more optical variety than a 28 alone - and I can't really think of a situation on FF where you'd want to take a 28 prime over a 24 or 35; on FF, most of my street photog is with the 35mm f/2 IS or 50L. Also, while the 24mm f/2.8 IS is one of the best landscape primes Canon makes (IMO better than the 24L), I mainly do events and portraiture where the higher speed of the 24L is frequently needed. But, the 28mm IS is a nice little lens that approaches the size of the 40mm pancake, while the 24mm IS is just a bit too long to be considered truly compact.

Another option - if you don't do much landscape - is to simply buy the 28mm f/2.8 IS for your crop, then sell it and keep the 17-40 for landscape when you go full frame (or upgrade the 17-40L to the new 16-35 f/4L IS). If you are into street photography, neither the 24/28 will be best for that on FF - probably better off with a 35mm f/2 IS USM at the very widest (which will give similar FOV to 24mm on crop). One of the best pictures street photog pictures I've ever taken was on the 35mm f/2 IS USM on a 6D - and that lens is very optically similar to the 24/28, just a stop faster.

Also as someone else noted, using the lens at 1/100-1/125 minimum shutter speed is crucial to prevent motion blur if you are taking pictures of people, at which point IS will become less needed due to the fast shutter on a shorter focal length.

Choose a PRIME only if you are regularly and often shoot with the same focal length. Otherwise, choose a ZOOM. Maybe the 16-35 f/4 IS L is a better choice?

I only use my 17-40L when I shoot architecture. 99.99% of the time I use my 50mm f/1.4 on my DSLR and my 40mm on my SLRs. Those focal lengths are way too cropped for the types of shots I want with my DSLR when I'm in Atlanta. I think getting a 16-35 f/4L IS would be redundant and too large for remaining discreet.

I was in a similar situation when these lenses were just released and opted for the 24mm IS. I really liked the look of 24mm on the APS-C sensor and the images came out super sharp. It can also still be good for street photography and definitely great for landscape if you decide to go FF later on. The 28mm is rated to be sharper, but may not be wide enough.

Today I went shooting in Atlanta and brought along my Canon Rebel GII 35mm camera. I attached my 40mm f/2.8 STM on it, knowing full well that 28mm is 44.8mm on crop sensor. It was simply amazing to shoot at that lens and camera combination for an extended period of time. It's wide enough for me. So, I'll be getting the 28mm f/2.8 IS USM.

Thanks for all of your help! When I get it, I'll update the thread with my impressions.

Today I went shooting in Atlanta and brought along my Canon Rebel GII 35mm camera. I attached my 40mm f/2.8 STM on it, knowing full well that 28mm is 44.8mm on crop sensor. It was simply amazing to shoot at that lens and camera combination for an extended period of time. It's wide enough for me. So, I'll be getting the 28mm f/2.8 IS USM.

Thanks for all of your help! When I get it, I'll update the thread with my impressions.

Please update with some photos, too. I love to watch street photography. Cheers!